WASHINGTON (AP) -- Satellite imagery indicates North Korea has been testing rocket engines, a sign it continues to develop its long-range ballistic missiles, a U.S. academic institute said Monday.

The analysis provided to The Associated Press is based on satellite images taken as recently as late September of the Sohae site on the secretive country's northwest coast. In April, the North launched a rocket from there in a failed attempt to propel a satellite into space in defiance of a U.N. ban.

The analysis on the website of the U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies, which is called "38 North," said it remains unclear whether the North is preparing another rocket launch but predicted it may embark on new rocket and nuclear tests in the first half of 2013.